Aqueous polyurethane dispersions are widely used as binders for aqueous paints. Among these paints there is knowledge of one-component systems, which dry physically (by evaporation of the water), the formation of a film being assisted where appropriate by addition of thermally activated crosslinking agents (baking varnishes, curing with melamine resins or blocked isocyanates at temperatures above 120 to 140° C.); and two-component systems, where hydroxy-functional binder constituents are mixed with nonblocked isocyanates immediately prior to application and the crosslinking reaction takes place at room temperature (or at an elevated temperature in order to accelerate the cure). In the latter case it is also possible to emulsify the nonblocked isocyanate in the aqueous dispersion of the hydroxy-functional binder component, without any premature reaction of the two components.
Physically drying polyurethane dispersions are obtained, conventionally, by chain extending an isocyanate-functional prepolymer in the aqueous phase by means of polyfunctional compounds of low molar mass dissolved in water, referred to as chain extenders, such as diamines or dihydrazides.
In the case of solvent borne physically drying systems of this kind, high-viscosity solutions with a relatively low solids content are applied. The paints based on such systems dry rapidly to give tough, elastic films which exhibit possessing good solvent resistance. These paints are used to coat flexible substrates such as synthetic leathers and, for example, magnetic tapes and magnetic foils.
Coatings obtained from the existing aqueous systems are still inferior to those from solvent borne systems.
The object is therefore to provide aqueous polyurethane dispersions which dry physically to give tough, elastic films with good solvent resistance.
This object is achieved by means of a particularly high molar mass, water-dispersible polyurethane resin.